The present invention relates to an apparatus and method for monitoring the clearance ("gap") between the refining surfaces of first and second relatively rotating metallic members for refining fibrous cellulosic material. More particularly, the invention relates to a means for determining the gap on the basis of the electrical impedance across the gap and producing an electrical signal to drive a control system which maintains the gap at a desired size.
Cellulosic fibers must be subject to mechanical treatment before they can be made into paper. This treatment may be applied in a number of different ways, but it generally includes a bruising, rubbing or crushing action on the fibers. The terms beating and refining are used in the paper industry to describe the operation of mechanically treating pulp fibers. Refining usually refers to a fiber separation and fiber cutting action.
In disc refining, two parallel discs rotate relative to one another with a space therebetween. The surfaces of the discs have refiner plates mounted thereto which provide a refiner surface and define the refiner gap. The refiner plates have a precise configuration of angled bars and grooves so that wood chips or pulp fed into the gap will be subjected to a refining action. The distance between the refiner plates (i.e., the gap size) and the pressure exerted on the material being refined can be regulated to vary the degree of refining action.
The material between the plates (e.g., wood chips or pulp) forms a "pad" which serves to prevent the opposed plate surfaces from contacting one another. Such contact, known as "plate clashing", destroys the precise configuration of angled bars and grooves of the opposed plates and causes a degradation in the quality of the refining action. If the damage to the plates is extensive, the disc refining apparatus must be shut down and the damaged plate segments must be replaced.
Various techniques for preventing plate clashing are known in the art. These techniques have been developed to eliminate the costly down-time which results from plate clashing, and to maximize the amount of product which is refined by the disc refiner. For example, in U.S. Pat. No. 2,548,599, clearance control between the two refining plates of a rotary disc pulp refiner is provided by monitoring a magnetic flux path wherein the reluctance is determined by the space between the plates. Another magnetic arrangement for use in measuring plate separation in disc refiners is disclosed in U.S. Pat. No. 3,434,670. In this patent, a plurality of sensing coils are spaced around the periphery of one of the discs and at least one magnet is mounted adjacent to the periphery of the other disc. Upon rotation of the discs relative to each other, current pulses are produced in the coils which have a value dependent upon the spacing between the sensing coil and the magnet.
Other clearance control systems which are known include that shown in U.S. Pat. No. 3,799,456 which utilizes a pair of linear displacement transducers to produce voltages related to the positions of refiner plate surfaces. The voltage outputs from the transducers are summed to produce a composite signal representing the distance betweeen the refining surfaces during a refining operation. It is also known to use ultrasonic measurement techniques for determining the adjustment of a crusher gap setting, as shown in U.S. Pat. No. 3,944,146.
U.S. Pat. No. 4,073,442 relates to an electrically controlled system for regulating the grinding space in a grinding apparatus. In this patent, the moisture in wood chips which are collected in the grinding space between two metallic grinding discs is utilized to produce a conductive coil. The resistance of this cell varies in response to fluctuations in the grinding space. In U.S. Pat. No. 3,133,707, a hydraulic shaft repositioning system is actuated in a gyratory crusher in response to a capacitive Wheatstone bridge circuit. Another capacitance control position indicator for a gyratory crusher is shown in U.S. Pat. No. 4,251,035.
Another gyratory clearance measuring means is disclosed in U.S. Pat. No. 3,436,654. In the apparatus of this patent, a foil is inserted between one of the crushing surfaces and the structure of the crusher. The foil is grounded and an electrical signal is supplied between ground and the crusher surfaces. The capacitance between the surfaces is measured by using a high frequency source and a measuring circuit connected to the surfaces through tuned resonant circuits. The apparatus disclosed in this patent requires the use of a separate foil electrode which is sandwiched between a fixed crushing surface and a moving crushing surface. The apparatus is not applicable to a situation where both crushing surfaces are moving.
It would be advantageous to provide a means for monitoring the gap spacing between two counter-rotating refiner discs. The apparatus to accomplish this should not require considerable installation costs and should be readily adapted for installation in existing disc refiners. Such an arrangement should provide an electrical output signal which is proportional to the gap between the counter-rotating discs without a requirement for modifying the existing equipment. The output signal should be capable of controlling, through an appropriate servo-system, the size of the gap.
The present invention relates to such an apparatus.